If you want 12 hrs at night then she can only have a 12 hr A time. So you would put her to bed at 6 and hope that she does at least 12 hrs. Just looking at it mathematically, anyway.
If she naps I'd do a 7 p.m. - you might still only get to 5 or 6 a.m. but at least she will have had a longer night. Or maybe she will surprise you and tack.
To push the day out and get out of the OT loop I think you would need to offer an earlier nap + later bedtime. Or like Heidi has suggested, work on cutting or losing the nap and getting to a 12 hr night.